This invention is generally in the field of high-temperature composite sealant compositions, and more particularly composite sealants useful in the fabrication of a solid oxide fuel cell (SOFC) and SOFC stacks.
A fuel cell is a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell typically includes a cathode material that defines a passageway for the oxidant and an anode material that defines a passageway for the fuel. An electrolyte is disposed between and separates the cathode and anode materials, and electrochemical conversion occurs at or near the three-phase boundary of the electrodes (cathode and anode) and electrolyte, producing a DC electrical output.
The fuel and the oxidant are fluids, usually gases, that are continuously passed through separate cell passageways. In a SOFC, hydrogen or a hydrocarbon is commonly used as the fuel, while oxygen or air is used as the oxidant. The SOFC electrolyte is in a solid form, typically a nonmetallic ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. The electrolyte provides a voltage buildup on opposite sides of the electrolyte, while isolating the fuel and oxidant gases from one another. The anode and cathode are generally porous, with the anode oftentimes being made of nickel/YSZ cermet and the cathode oftentimes being made of doped lanthanum manganite.
An individual SOFC cell usually generates a relatively small voltage. Thus, the individual electrochemical cells are connected together in series to form a stack, in order to achieve higher, useful voltages. Electrical connection between cells is attained with an electrical interconnect between the cathode and anode of adjacent cells. In addition, the stack usually includes ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
Whether in the form of a stack or individual cells, it is important that the fuel and oxidant be kept separate from one another in order to avoid or minimize efficiency losses in producing the exchange of ions across the electrolyte and to decrease or eliminate the potential for explosions. Consequently, sealants are needed to seal different portions of a SOFC stack, in particular at the edges and where manifolds must be connected to the stack. SOFC sealants, however, remain one of the outstanding materials challenges, as the sealant must meet several, often competing, property requirements.
Generally, the sealant should exhibit the following characteristics: (1) a relatively high coefficient of thermal expansion (CTE) to match that of the stack components, including the cells, interconnects, and manifold materials, which components typically have CTEs ranging from about 9 to 15×10−6/° C. at 800° C.; (2) a relatively low softening point for thermal stress relief during system shut-down and start-up; (3) a desirable viscosity value, such that the sealant is sufficiently fluid to seal gaps at the sealing/assembling temperature and be viscous enough at the cell operating temperature (typically 700-900° C.) so that gaps are kept sealed under gas pressure differentials; (4) thermally and chemically stable (e.g., negligible phase changes, negligible weight loss, minimum reaction with stack and manifold materials, and minimum ions migration under the electrical field) in the SOFC operating environment and conditions; (5) easily applied between surfaces and able to fill gaps between non-parallel surfaces. These properties are very difficult to satisfy simultaneously.
Various sealant compositions have been made with an effort to achieve the sealant characteristics described above. However, these compositions have their own shortcomings or limitations. For example, U.S. Pat. Nos. 4,774,154 and 5,110,691 disclose sealant compositions that are unsuitable for use at SOFC operating temperatures greater than 400° F. Another example is found in U.S. Pat. No. 6,271,158, which discloses a composite sealant using a glass matrix, which accomplishes the sealing, and one or more fillers that modify the CTE and improve the gap-filling capacity of the composite material. U.S. Pat. No. 6,541,146 also describes sealant materials comprising glasses containing alkali ions. The compositions, however, includes as one of the primary ingredients glasses containing alkali ions (e.g., potassium and sodium). These alkali ions may undesirably tend to migrate towards the anode under electrical field as low as 0.7 volts, which exist in operating fuel cells. Such an ion migration over time causes microstructure defects such as voids, porosity and even bubbling in the sealant, which undermines the long-term stability and integrity of the seal. The patent also discloses using metallic fillers, which at high loading yield undesirable electrical conduction paths, which may provide current leakage detrimental to seal integrity. It therefore would be desirable to provide improved sealant materials that minimize ions migration issues and which satisfy other property requirements of fuel cell sealants.